Pressure-sensitive adhesion properties or intrinsic adhesion properties are not typical of polyurethanes. Although polyurethanes occupy position five in the list of plastics, in terms of the amounts produced, PSAs of this material play only a very minor role economically.
Nevertheless, polyurethane PSAs have been known for a long time and are described diversely.
The effect of pressure-sensitive adhesiveness can be obtained by adding tackifier resins and/or plasticizers to the polyurethane base polymer. This method is described in, for example, U.S. Pat. No. 3,437,622 A (Dahl et al., Continental Tapes), U.S. Pat. No. 3,718,712 A (Tushaus et al., 3M), U.S. Pat. No. 4,087,392 A (Hartmann et al., BASF), DE 19 04 102 A (Hagenweiler, BASF) and JP 2000 256 639 (Toyo).
PSAs of this kind generally have the drawback of not attaching selectively only to certain surfaces. Moreover, following a prolonged period of bonding, they unite—usually strongly—with the surfaces and therefore often cannot be removed.
This “uniting” with the surface is often described by a person skilled in the art as “peel increase”, to describe the increase in bond strength during storage of the adhesive bond. It is also possible for the tackifier resin to migrate into the bonded surfaces, where it leaves behind spots which have a greasy appearance.
High peel increase on numerous surfaces, the resultant difficulty of redetachment, and the tendency to leave behind spots with a greasy appearance on many surfaces, are observed not least for those PSAs where the pressure-sensitive adhesiveness is obtained by undercrosslinking: i.e. an amount of isocyanate groups in deficit to the isocyanate-reactive groups, such as hydroxyl or amino groups, for example.
PSAs designed on the principle of undercrosslinking are described for example in U.S. Pat. No. 5,157,101 A (Orr, Norwood), DE 24 35 218 A (Adsley et al., Adhesive Tapes), JP 59 227 922 (Sanyo), U.S. Pat. No. 3,930,102 A (Szonn et al., Beiersdorf), U.S. Pat. No. 5,714,543 A (Kydonieus et al., Bristol Myers Squibb), EP 0 597 636 A1 (Kydonieus et al., Squibb) and U.S. Pat. No. 5,591,820 A (Kydonieus et al., Squibb).
Polyurethane PSAs containing monools fall into a very similar category with analogous weaknesses. Polyurethanes of this kind are likewise undercrosslinked and therefore contain relatively large fractions of migratable polyurethane units of low molecular weight. Polyurethane PSAs on this basis are known for example from EP 0 882 749 A1 (Ikeda et al., Nitto), U.S. Pat. No. 5,227,409 A (Mobley et al., Dow) and U.S. Pat. No. 5,102,714 A (Mobley et al., Dow).
Another type of polyurethane PSAs uses polyol components which carry hydroxyl groups and contain double bonds. Polyurethane PSAs on this basis are set out for example in JP 02 003 476 (Tsubota et al., Shinko), WO 98/30648 A1 (Gerard et al., Shell), JP 59 230 076 (Sekisui), JP 2001 146 577 (Toyo), U.S. Pat. No. 3,879,248 A (Kest), U.S. Pat. No. 3,743,616 A (Kest), U.S. Pat. No. 3,743,617 A (Kest), U.S. Pat. No. 5,486,570 A (St. Clair, Shell) and U.S. Pat. No. 3,515,773 A (Dahl et al., Continental Tapes). A drawback is the oxidative sensibility of these PSAs, caused by the double bonds in the polymer main chain. After a certain time this leads to filming or to “blunting” of the pressure-sensitively adhesive surface. In addition the majority of PSAs of this type additionally contain resins, with the disadvantages already described earlier on above.
A special polyurethane PSA containing carbon-carbon double bonds and based on castor oil, a natural product, is described in U.S. Pat. No. 3,246,049 A (Webber, Norton). Here again the oxidative sensitivity is to be regarded as a weakness.
EP 0 979 835 A (Questel et al., Elf Atochem) proposes hydroxyl-terminated polyalkylenes as a polyol component, which will solve the problem of oxidative sensitivity. The compositions, however, are moisture-curing, consequently obtain a high ultimate cohesive strength and cannot be used more than once, so that they are unsuitable for reversible adhesive bonding of paper. Furthermore they contain tackifier resins and plasticizers, whose disadvantages have already been described earlier on above.
Moisture-curing polyurethane PSAs are also described in, for example, U.S. Pat. No. 4,661,542 A (USM), JP 63 189 486 (Sanyo) and EP 0 196 749 A1 (von Voithenberg et al., Emhart).
A polyurethane PSA based on hydrogenated polybutadienes is described in JP 01 156 386 (Uehara et al., Hitachi). A drawback there is the need for electron beam crosslinking, which involves a considerable level of technical complexity.
A polyurethane PSA likewise requiring electron beam curing is known from JP 63 260 917 (Uehara et al., Hitachi). It uses polyethers as a polyol component.
Certain publications describe polyurethane-including blends or polyurethane copolymers having pressure-sensitive adhesive properties. Examples include U.S. Pat. No. 5,910,536 A (Kydonieus et al., Bristol Myers Squibb), U.S. Pat. No. 5,714,543 A (Shah et al., Bristol Myers Squibb) and U.S. Pat. No. 4,626,475 A (Barnett et al., Ashland Oil). These PSAs generally feature a heightened tack and are therefore difficult to remove from sensitive substrates without damaging them. As a general rule their pressure-sensitive adhesiveness is not limited selectively to particular surfaces.
Polyurethane PSAs having special additional properties, such as flame retardancy or electrical conductivity, for example, are described in, for example, EP 1 108 768 A1 (Wong, Tyco) or U.S. Pat. No. 4,855,077 A (Hata et al., Takiron).
Foamed polyurethanes having pressure-sensitive adhesive properties are likewise known. An example that may be mentioned is the publication DE 24 35 217 A (Adsley et al., Adhesive Tapes), and also the descriptions of hydrophilic foams in DE 42 33 289 A (Kenndoff et al., Beiersdorf) and WO 94/07935 A (Kenndoff et al., Beiersdorf).
In principle, as a result of the enlargement of surface area, foamed polyurethanes have the drawback of a heightened oxidative sensitivity and also of a heightened light sensitivity. In practice it has been found that they exhibit strong peel increase on the majority of surfaces and either cannot be removed without damage or else, particularly in the case of the foams made hydrophilic by additions of superabsorbent, interact with the substrate in such a way that spotting occurs.
Polyurethanes having pressure-sensitive adhesive properties can also be obtained, as demonstrated in JP 2000 073 040 (Toyo) and JP 2000 256 638 (Toyo), by using not only polyethers but also polyesters and also two different catalysts within a polyol component formula. A particular drawback in this case is the increased preparation complexity resulting from the formulas.
JP 2000 328 034 (Toyo), U.S. Pat. No. 3,761,307 A (Dahl) and U.S. Pat. No. 3,925,283 A (Dahl, Continental Tapes) describe pressure-sensitive adhesive polyurethane/-ureas which are obtained by incorporating additional amine-type chain extenders or crosslinkers into the polymer. Drawbacks perceived are the complexity in preparation and the assumed low selectivity of the pressure-sensitive adhesiveness on different surfaces.
DE 21 39 640 A (Dollhausen et al., Bayer) describes a PSA based on an aromatic diisocyanatourethane. A particular drawback is the yellowing tendency, which is typical of aromatic polyurethanes.
In order to achieve pressure-sensitive adhesion properties DE 100 30 908 A (Bolte et al., Henkel) and EP 0 081 103 A1 (Miyake et al., Takeda) proposed using two different isocyanates within a polyurethane composition. In these cases too the complexity in preparation is found to be a drawback, as is the low selectivity of the pressure-sensitive adhesiveness on different surfaces.
WO 97/22642 A1 (Chang et al., Bristol Myers Squibb) proposes, for the preparation of a PSA, heating an NCO-terminated prepolymer and a polyhydroxy compound together at a certain temperature until a gel fraction of 30 to 40% is obtained. A disadvantage of this method is the assumed low selectivity of the pressure-sensitiveness on different surfaces, which results from the relatively low gel content.
U.S. Pat. No. 3,796,678 A (Bartizal, 3M) discloses a polyurethane PSA based on capped isocyanate prepolymers which relies on water or organic solvents for its preparation. The complex nature of the preparation is regarded as a drawback, along with the unavoidable need to use water or solvents.
A polyurethane latex PSA is described in WO 98/31760 A1 (Schrock et al., Dow Chemical). A drawback is the need for drying, which makes it either impossible or at least very time-consuming to obtain blister-free, relatively thick PSA films.
Certain publications define a polyurethane PSA by way of the crosslinking density. GB 1,113,925 A (Weller) and GB 1,216,672 A (Grindley) propose chain lengths of 130–285 chain atoms and, respectively, more than 285 chain atoms between the crosslinking points. In practice it has been found that controlling the PSA properties by way of the criterion of chain length alone is an impossibility. An insufficient crosslinking density results in severe peel increase on the majority of substrates after a prolonged bonding period, while too high a crosslinking density leads to PSAs whose pressure-sensitive adhesiveness is inadequate. Surface-specific selectivity of the PSA properties is not attained.
EP 1 088 871 A1 (Heguri et al., Sekisui) prescribes a certain distance between the isocyanate groups, or a certain degree of crosslinking, for the polyisocyanate used. The molecular weight between two isocyanate groups in the polyisocyanate is intended to amount to from 220 to 570. This way of controlling the crosslinking density by way of the chain length within the polyisocyanate is likewise unlikely to improve the surface-specific selectivity of the PSA properties.
In U.S. Pat. No. 6,040,028 A (Cline et al., Bayer) as well a polyurethane adhesive (contact adhesive) is defined by way of the molecular weight between crosslinking points. A molecular weight of between 7000 and 16000 is prescribed. Further restrictions are imposed, to the effect, inter alia, that from 0 to 10% of the polyols must have a molecular weight of from 60 to 400 and from 90 to 100% of the polyols must have a molecular weight of from 1800 to 12000.
Here again it must be assumed that the PSA properties are not selectively surface-specific and that on the majority of substrates there will be a sharp peel increase following prolonged adhesive bonding, since the crosslinking density is relatively low and no indications of a possible solution to the problem are given.
WO 01/62818 A1 (Hansen et al., 3M) proposes reacting two polyols or other NCO-reactive materials with isocyanates for the preparation of a polyurethane PSA, the isocyanate-reactive components differing from one another in that one has a molecular weight of more than 2000 and one has a molecular weight of less than 2000.
PSAs of this kind are known inter alia from the examples in U.S. Pat. No. 5,227,409 A and U.S. Pat. No. 3,437,622 A, which also includes the restriction (claim 5) whereby the isocyanate-reactive components are almost exclusively diols. Additionally, polyurethanes with such a composition have already been described in the examples in EP 1 095 993 A1 and EP 1 101 803 A1, the last-mentioned examples not relating to compositions intended for the obtainment of PSAs. The proposed reaction product in WO 01/62818 A1 is therefore not definitively a PSA. There are no references to a surface-specific selectivity of the PSA properties.
It is an object of the invention to provide a pressure-sensitive adhesive which adheres well to polar plastics surfaces and smooth metal surfaces and also to glass, which in particular is easy to remove, without damaging surfaces during its detachment, particularly when the surfaces are sensitive to mechanical influences, and which does not display the above-described drawbacks of the prior art, or not to the same extent. In particular it should show substantially no adhesion or only very little adhesion to paper, paperboard or similar materials and also to human skin, should essentially not feel tacky, should not leave any residues or spots of greasy appearance following its removal, and should also be able to be used more than once, removed by washing and stable to light.